Wall filter

The wall filter in ultrasound is a way of filtering out low or high frequency Doppler signals. In clinical ultrasound, it is usually used to filter out very low frequencies that may add noise to a spectral Doppler waveform.

A typical use is removing the low frequency reverberation of an arterial wall. The arterial wall motion may produce an audible (and visible) "thump", and this can interfere with the spectral Doppler waveform of the arterial blood flow. The wall filter removes these low frequencies.

A potential pitfall is that the wall filter may filter out genuine low-frequency slow flow, such as venous blood flow. If the goal is to evaluate venous blood flow, make sure the wall filter is set as low as possible.